The campaign has been riddled with claims of voter suppression efforts against likely Abrams supporters. Abrams has called for Kemp to resign.

In a statement, Kemp’s office said a “failed attempt to hack the state’s voter registration system” had led to the investigation being opened on Saturday evening. Federal authorities were alerted. Candice Broce, Kemp’s press secretary, said “no personal data was breached” and “our system remains secure”.

Sign up for the new US morning briefing

On Sunday morning, Abrams appeared on the political talk shows. Asked about the investigation on CNN’s State of the Union, she said: “I’ve heard nothing about it.”

Referring to legal verdicts in cases concerning alleged voter suppression, she said: “This is a desperate attempt on the part of my opponent to distract people from the fact that two different federal judges found him derelict in his duties and forced him to allow absentee ballots to be counted and those being held captive by the exact match system to be allowed to vote.

“He is desperate to turn the conversation away from his failures from his refusal to honour his commitments and from the fact that he is part of a nationwide system of voter suppression that will not work in this election. Because we are going to outwork him we are going to outvote him and we are going to win.”

Georgia: voter suppression allegations roil governor's race

Read more

Polling in the race is extremely close, most analysts rating the outcome a toss-up. Big names have duly been attracted to the state: Donald Trump is due to stage a rally in Macon on Sunday, an event which prompted Kemp to withdraw from a scheduled debate. Barack Obama and Oprah Winfrey have stumped for Abrams.

Play Video

1:37

'Vote your values': Oprah Winfrey campaigns for Stacey Abrams - video

If no candidate wins more than 50% of the vote, a runoff vote will be staged on 4 December.

Kemp did not immediately comment on the new investigation. Among tweets from his official account, he linked Abrams to the New Black Panther party as “too extreme” for Georgia.

On Saturday, it was reported that racist robocalls bankrolled by white supremacists were targeting Abrams and Winfrey.